1. Field of the Invention
The present invention generally relates to automatic controls for heating equipment and, more particularly, to automatic tuning of multi-zone furnaces.
2. Description of the Prior Art
The manufacturing processes for many types of devices and materials include heat treatment processes. These processes may include raising the device or material to a predetermined temperature for a predetermined period of time or treatment of the device or material at a sequence of different temperatures for respective different periods of time.
It is also often desirable in such manufacturing processes, to provide for continuous processing of such devices or materials, particularly where a sequence of different temperatures are to be used. Furnaces for continuous production often take the form of an elongated chamber which is divided into a plurality of zones which may be maintained at different temperatures. Such a furnace, used for cooking, is shown in U.S. Pat. No. 4,554,437, to Wagner et al. The furnace may include a muffle, generally of circular or elliptical cross-section, to assist in regulating difference in temperature from zone to zone and confine the atmosphere in the vicinity of the heated material as taught in U.S. Pat. No. 4,416,623, to Takahashi. In such arrangements, it is common to place temperature sensors at the muffle in proximity to each heating segment to regulate the muffle temperature and to avoid deformation thereof. The devices or material are then transported therethrough on a conveyor-like arrangement at a speed which assures the desired amount of heat treatment at each temperature. This is referred to as a continuous furnace. Also, in such arrangements, certain zones can be maintained at intermediate temperatures in order to "ramp up" or "ramp down" from one temperature to another.
Ovens of similar multi-zone construction may also be used for batch processing where the oven is initially charged with material and the heat treatment carried out without moving the material. In such a case, the conveyor arrangement may be omitted and all zones maintained at the same temperature, although temperature could be selectively changed as a function of time during the heat treatment process. In this case, the avoidance of temperature gradients is even more critical than in the case of the continuous furnace which will at least perform heat treatment equally on all material passed therethrough.
It can be readily appreciated that such multi-zone furnaces are potentially far more efficient than a furnace where the device or material remained stationary and the temperature of the furnace varied. The consistency of the heat treatment is inherently more uniform and the thermal mass of the furnace itself does not slow heating or cooling when temperature is to be altered, allowing throughput of the furnace to be maximized. Further, since the temperature of each zone ideally remains substantially constant, no energy is wasted in altering the temperature of the thermal mass of the furnace itself.
Multi-zone furnaces, however, are inherently large due to the number of zones which may be provided. The volume of each zone must be comparable to the volume of a single furnace which would be suitable for the device or material to receive heat treatment and may be advantageously made larger to provide for a more smooth temperature transition between zones. Typically, the atmosphere within the furnace will be able to circulate between zones and within each zone and substantial undesirable temperature gradients may occur between or within zones, due to convection and other heat transfer mechanisms. It is therefore known to provide for automatic control of furnaces intended for use where such temperature gradients may be critical, such as in the processing of semiconductor wafers or the manufacture of electronic components such as multi-layer ceramic (MLC) modules. Such an arrangement is shown in U.S. Pat. No. 4,886,954, to Yu et al., which shows a plurality of sensors, digital signal mixing and temperature computation and the use of a so-called PID algorithm (Proportional-plus-Integral-plus-Derivative) to develop signals for control of each heater element of the furnace. Generally speaking, a PID algorithm uses a measured value and an operating set point to derive control for a variable in such a way as to simultaneously maximize the response time for correction and the stability of the automatically controlled system. The details of operation of such an algorithm are not necessary to an understanding of the invention However, additional information concerning PID algorithms and use of the same may be found in Control System Principles and Design, by Ernest O. Doebelin, pp. 422-436, John Wiley & Sons, publisher and Control Systems - Analysis, Design and Simulation, by John W. Brewer, pp. 199-202, Prentice-Hall, Inc., Publisher, both of which are hereby incorporated by reference.
Most furnaces will have heating elements or "segments" placed around the periphery of the muffle in which heat treatment is to be performed and each segment will be controlled either individually or together with one or more of the other segments. U.S. Pat. No. 4,886,954, cited above, is exemplary of such arrangements. To provide inputs to the control arrangement including a PID algorithm, sensors are typically placed near the muffle and in proximity to the heater segment to be controlled. These segments will be subjected to thermal and mechanical stresses during use as they are turned on and off or otherwise controlled to maintain a nearly constant temperature. The segments will be subjected to further mechanical stresses in the form of vibration due to the means for transporting devices or materials through the furnace. Therefore, occasional failure of a segment is to be anticipated.
However, the failure of even a single segment will require shutting down the furnace in order to replace the segment, resulting in loss of productivity. The likelihood of interruption of operation for such repairs is increased by the number of segments which are present and increases with the number of zones in the multi-zone furnace. Moreover, the economic loss may not be limited to the "down time" of the furnace where, for instance, in a large run of electronic components which must be processed under highly uniform conditions to minimizes chip-to-chip variations, failure of a single heater segment could cause catastrophic loss of an entire run of the components.
Also, in the prior art, it should be noted that one or more sensors may be provided with a PID algorithm (hereinafter simply PID) for each heater segment. The sensors, which are typically thermocouples, are usually located in some proximity to the heater segment since they cannot be placed in the same location as the material or device to be treated. Therefore, each combination of heater segment, sensor group and PID typically operates autonomously from other combinations of heater, sensors and PID. Consequently, it can be understood, as theorized by the inventors, that the more closely the temperature of the furnace is controlled, the greater the number of control cycles will be, which may contribute to degradation of the heater segments and early failure thereof.
For example, as taught by Yu, U.S. Pat. No. 4,886,954, it is also known to provide separate temperature control systems for heater segments at the top and bottom of the furnace in order to reduce the top-to-bottom temperature gradient in the furnace. As disclosed therein, when the top and bottom are commonly heated, the temperature gradient is more pronounced at lower temperatures where heat transfer by convection will be of greater significance in comparison to radiation. This would also occur if the top and bottom sections (each comprising one or more heater segments) were independently controlled relative to a common set point. To minimize the temperature gradient, the system of Yu, U.S. Pat. No. 4,886,954, uses the sensed temperature at the bottom of the furnace both to control the bottom segments and as a set point for the top segments. While this technique was evidently effective to reduce top-to-bottom temperature gradients, such slaving of the top heater segments to the bottom will also tend to maximize the difference between duty cycles of the top and bottom heaters. For example, if the bottom sensor senses a temperature below the set point, the bottom heater segments will be turned on, producing convection and increasing the temperature sensed by the temperature sensors at the top of the furnace. At the same time, the PID for the top heater segments will be controlled to a lower set point and be held off in conformance with the goal of reducing the top-to-bottom temperature gradient. It can be seen that during periods of increasing the furnace temperature, with which Yu, U.S. Pat. No. 4,866,954, is concerned, the heating will be principally done by the bottom heater segments. Therefore, while this arrangement may be successful in reducing temperature gradients, it also tends to maximize the differences in conditions which lead to decay (e.g. thermal aging) and failure of the heater segments.